1. Field of the Invention
The present invention concerns a birdcage resonator for magnetic resonance applications.
2. Description of the Prior Art
Birdcage resonators for magnetic resonance apparatuses are generally known. They are normally constructed as follows.
The birdcage resonator has a first ferrule and a second ferrule disposed in ferrule planes parallel to one another and spaced from one another. The ferrules exhibit ferrule centers. A connection line of the ferrule centers with one another defines an antenna axis of the birdcage resonator which orthogonally intersects the ferrule planes. The birdcage resonator furthermore has a number of antenna rods that are distributed regularly (normally uniformly) around the antenna axis. Each antenna rod extends from the first ferrule to the second ferrule.
In magnetic resonance systems, birdcage resonators are used almost without exception as transmission antennas for excitation of magnetic resonances. Furthermore, they are also used as whole-body reception antennas for magnetic resonance systems as well as, in some cases, as local coils (for example as head coils). The reason for their use is in particular that birdcage resonators exhibit clear advantages with regard to the design and properties relative to possible alternative solutions.
In birdcage resonators, capacitors are arranged in the antenna rods and/or in the ferrules, the birdcage resonator being tuned by means of those capacitors. Depending on the arrangement of the capacitors, the birdcage resonator is designed as high-pass, as low-pass or as band-pass.
One of the disadvantages of birdcage resonators is what is known as ferrule modes, meaning currents that oscillate in the ferrules. The ferrule modes are parasitic resonances. They can be influenced only to a limited extent in birdcage resonators of the prior art. In the prior art the ferrule modes are disadvantageous since, in individual cases, they can be disruptive for the magnetic resonance imaging. A utilization of the ferrule modes is not known in the prior art.
In the prior art it is sought to place the frequencies of the ferrule modes in frequency ranges that are sufficiently far removed from the operating frequencies of the birdcage resonator, thus normally the frequencies with which currents oscillate in the antenna rods. This is not always possible, however. Due to other boundary conditions that must be achieved to depending on the circumstance of the individual case.
U.S. Pat. No. 4,885,539 discloses a birdcage resonator of the type described above that has a first coupling ring and a second coupling ring in addition to the typical elements described in the preceding. The first coupling ring proceeds around the antenna axis in a first coupling ring plane orthogonal to the antenna axis and is spaced from the first ferrule, but couples therewith. The second coupling ring proceeds around the antenna axis in a second coupling ring plane orthogonal to the antenna axis and is spaced from the second ferrule, but couples therewith. The coupling rings are tuned such that the ferrules are resonant at different frequencies than without the coupling rings. A similar disclosure can be learned from U.S. Pat. No. 6,396,271.
A birdcage resonator is known from U.S. Pat. No. 6,316,941, in which the frequency of one of the ferrules modes coincides with the frequency of one of the other modes and is utilized for reception of magnetic resonance signals.
It is possible that the ferrules are spaced so far from one another that they do not couple with one another, but the ferrules normally couple with one another such that, as a common eigenmode, they exhibit a common mode with a common mode frequency and a push-pull mode with a push-pull frequency differing from the common mode frequency.